This invention relates generally to the field of microscopic spectrographic analysis of structures of materials and more particularly to a method and apparatus for scanning tunneling optical spectroscopy and microscopy suitable for measuring properties of semiconductor materials, buried semiconductor structures and other sub-surface features of photoconductive materials.
Scanning tunneling microscopes are known in the art for providing the spatial resolution and analytic capabilities to image atoms on the surface of solid materials and permit study of a variety of material properties on the atomic scale. In a scanning tunneling microscope, a sharply pointed conductive tip and a sample surface are brought very close together, on the order of approximately a nanometer apart, and a voltage is applied across the gap between the tip and the sample surface. A tunneling current flows which is extremely sensitive to changes in gap width. The tip is scanned over the surface and a feed-back system controls the distance of the tip from the surface using the deviation of the tunneling current as the control signal. This control signal is also employed to generate a plot of the topology of the sample surface.
Such scanning tunneling microscopes have been utilized in conjunction with optical sources to study the interaction of light with solids on an atomic scale. With one technique, deformations of the sample caused by heating due to the radiation of the sample with monochromatic light is detected using a tunneling microscope structure. In this way, optical absorption properties of the sample material are measured. In another approach, scanning tunneling microscope techniques are used to measure thermoelectric junction potential between a measurement tip and the sample in response to differential absorption heating of the sample with optical radiation. Optical radiation of the tip-sample junction is also used to enhance conductivity of the sample surface for photoconductive materials, or to generate surface photo-voltage effects resulting from formation of excess electrons and holes. None of these prior art systems scans the frequency of the optical source applied to the surface. In addition, none utilize modulated optical radiation together with phase sensitive detection of the resultant modulated photoexcited tunneling current to isolate the photoexcited tunneling current. Consequently, none of these prior art techniques are capable of detecting and analyzing spectral response of photoexcited tunneling current, thereby permitting band gap and sub-band gap transition effect measurements in a semiconductor; nor can they detect such semiconductor structures as buried structures including heterojunctions, homeojunctions, and quantum wells.
Accordingly, it is an object of the invention to provide a novel scanning tunneling optical spectrometer suitable for measuring properties of semiconductor structures, including buried structures such as quantum wells, homeojunctions and heterojunctions.
It is another object of the invention to provide a novel method of scanning tunneling optical spectroscopy capable of detecting the spectral response of photoexcited tunneling current in materials such as semiconductors and other photoconductive organic or inorganic materials.
It is another object of the invention to provide a novel method and apparatus for scanning tunneling optical spectroscopy capable of detecting buried semiconductor structures and of determining a local band gap of semiconductor material.
It is another object of the invention to provide a novel method and apparatus for performing scanning tunneling optical spectroscopy wherein the optical source performs a frequency scan over a range of frequency to determine the spectral response of photoexcited tunneling current at the probe-sample junction.
It is another object of the invention to provide a novel method and apparatus for scanning tunneling optical microscopy capable of generating a photoexcited current image of a sample for such materials as semiconductors or other organic and inorganic photoconductive materials permitting imaging of sub-surface structures.
Briefly, according to one embodiment of the invention, a scanning tunneling optical spectrometer is provided for measuring the properties of a sample and includes a mounting stage for mounting the sample. A fine pointed probe is positionable adjacent the sample and sufficiently close to the sample to permit flow of tunneling current between the sample and the probe with an electrical bias circuit connected to the probe and the sample to generate a bias tunneling current. An optical source for radiating the sample with monochromatic radiation at a sequence of optical frequencies modulated with at least one selected modulating frequency to generate photoexcited tunneling current which is detectable without substantial interference by tunneling current variations caused by thermal heating of the sample by the radiation. Circuitry for modulating the monochromatic optical radiation with at least one selected modulating signal is provided to improve signal to noise ratio, and a detection means is provided to detect the photoexcited tunneling current substantially independent of the bias tunneling current and without substantial interference from tunneling current variations caused by thermal heating of the sample by the radiation. In another embodiment, the probe is scanned over at least a portion of the surface and the detected photoexcited tunneling current is utilized to generate an image of the sample permitting scanning tunneling optical microscopy of surface and sub-surface features.